fix(queryCursor): add _transformForAsyncIterator transform after user-defined transforms

lib/cursor/queryCursor.js

@@ -77,9 +77,6 @@ function QueryCursor(query) {
     if (this.options.transform) {
       this._transforms.push(this.options.transform);
     }
-    if (this._mongooseOptions._transformForAsyncIterator) {
-      this._transforms.push(_transformForAsyncIterator);
-    }
     // Re: gh-8039, you need to set the `cursor.batchSize` option, top-level
     // `batchSize` option doesn't work.
     if (this.options.batchSize) {
@@ -382,27 +379,6 @@ QueryCursor.prototype.addCursorFlag = function(flag, value) {
   return this;
 };
 
-/*!
- * ignore
- */
-
-QueryCursor.prototype.transformNull = function(val) {
-  if (arguments.length === 0) {
-    val = true;
-  }
-  this._mongooseOptions.transformNull = val;
-  return this;
-};
-
-/*!
- * ignore
- */
-
-QueryCursor.prototype._transformForAsyncIterator = function() {
-  this._mongooseOptions._transformForAsyncIterator = true;
-  return this;
-};
-
 /**
  * Returns an asyncIterator for use with [`for/await/of` loops](https://thecodebarbarian.com/getting-started-with-async-iterators-in-node-js).
  * You do not need to call this function explicitly, the JavaScript runtime
@@ -435,18 +411,11 @@ QueryCursor.prototype._transformForAsyncIterator = function() {
 
 if (Symbol.asyncIterator != null) {
   QueryCursor.prototype[Symbol.asyncIterator] = function() {
-    return this.transformNull()._transformForAsyncIterator();
+    this._mongooseOptions._asyncIterator = true;
+    return this;
   };
 }
 
-/*!
- * ignore
- */
-
-function _transformForAsyncIterator(doc) {
-  return doc == null ? { done: true } : { value: doc, done: false };
-}
-
 /**
  * Get the next doc from the underlying cursor and mongooseify it
  * (populate, etc.)
@@ -457,16 +426,37 @@ function _transformForAsyncIterator(doc) {
 
 function _next(ctx, cb) {
   let callback = cb;
-  if (ctx._transforms.length) {
-    callback = function(err, doc) {
-      if (err || (doc === null && !ctx._mongooseOptions.transformNull)) {
-        return cb(err, doc);
+
+  // Create a custom callback to handle transforms, async iterator, and transformNull
+  callback = function(err, doc) {
+    if (err) {
+      return cb(err);
+    }
+
+    // Handle null documents - if asyncIterator, we need `done: true`, otherwise just
+    // skip. In either case, avoid transforms.
+    if (doc === null) {
+      if (ctx._mongooseOptions._asyncIterator) {
+        return cb(null, { done: true });
+      } else {
+        return cb(null, null);
       }
-      cb(err, ctx._transforms.reduce(function(doc, fn) {
+    }
+
+    // Apply transforms
+    if (ctx._transforms.length && doc !== null) {
+      doc = ctx._transforms.reduce(function(doc, fn) {
         return fn.call(ctx, doc);
-      }, doc));
-    };
-  }
+      }, doc);
+    }
+
+    // For async iterator, convert to {value, done} format
+    if (ctx._mongooseOptions._asyncIterator) {
+      return cb(null, { value: doc, done: false });
+    }
+
+    return cb(null, doc);
+  };
 
   if (ctx._error) {
     return immediate(function() {

test/query.test.js

@@ -2358,7 +2358,7 @@ describe('Query', function() {
     const Model = db.model('Test', new Schema({ name: String }));
 
     await Model.create({ name: 'test' });
-    const cursor = Model.find().transform(doc => doc?.name.toUpperCase() ?? null).cursor();
+    const cursor = Model.find().transform(doc => doc.name.toUpperCase()).cursor();
     const names = [];
     for await (const name of cursor) {
       names.push(name);